The present invention relates to a method for manufacturing a semiconductor device, in which an active layer is located on a supporting substrate with an insulating intermediate layer therebetween and a movable unit included in the active layer moves in relation to the supporting substrate in response to a force applied to the movable unit, which is correlated to a dynamic quantity to be measured by the device.
As such a device, a capacitive semiconductor acceleration sensor shown in FIGS. 4 and 5 is proposed. As shown in FIG. 5, the proposed acceleration sensor includes a substrate 110, which has a Silicon-On-Insulator (SOI) structure. The substrate 110 is composed of an active layer 112, an insulating intermediate layer 113, and a supporting substrate 111. The active layer 112 and the supporting substrate 111 are made of silicon, and the insulating intermediate layer 113 is made of silicon oxide.
As shown in FIGS. 4 and 5, the active layer 112 includes a movable unit 120 and two fixed units 130. The movable unit 120 is composed of a weight 121, two comb-shaped movable electrodes 124, and two rectangular springs 122. The movable electrodes 124 are joined to the weight 121. In addition, the springs 122 are joined to the weight 121. The movable unit 120 moves in relation to the supporting substrate 111 in response to an acceleration of the sensor. Each fixed unit 130 includes a comb-shaped fixed electrode 132. The fixed electrodes 132 are stationary in relation to the supporting substrate 111 under the acceleration of the sensor. As shown in FIG. 4, each of the fixed electrodes 132 interleaves with each of the movable electrodes 124 to form two capacitances.
The movable electrodes 124 move along the directions X of FIG. 4 with the weight 121 in response to an acceleration of the sensor along the directions X. When the movable electrodes 124 move, the clearances between the movable electrodes 124 and the fixed electrodes 132 change. The two capacitances are correlated to the clearances, so the two capacitances change in response to the acceleration. In addition, the two capacitances change in a manner that one of the capacitances increases while the other decreases. Therefore, the acceleration along the directions X can be measured based on the difference between the capacitances.
As shown in FIGS. 6A and 6B, in a conventional manufacturing process of the proposed acceleration sensor, a plurality of trenches 114, 114a, 114b that extend through a silicon layer 112, from which the active layer 112 is formed, to the insulating intermediate layer 113 are formed by dry etching the silicon layer 112. Then, the sidewalls defining the trenches 114, 114a, 114b are dry etched at the portions adjacent to the bottoms of the trenches 114, 114a, 114b to complete the movable unit 120 and the fixed electrodes 132, as shown in FIG. 6C.
Under the acceleration of the sensor, the clearances are determined by the movement of the movable unit 120, and the movement of the movable unit 120 is determined by the deformability of the springs 122, which are formed by the above dry etching steps. Therefore, increasing machining precision, or dry etching precision, of the springs 122 is essential to decrease sensor-to-sensor deviation in sensor characteristics.
In the conventional manufacturing process, the trajectory angle of etching ions are substantially orthogonal to the surface of the silicon layer 112 until the insulating intermediate layer 113 is exposed at the bottom of the trenches 114, 114a, 114b during the step of forming the trenches 114, 114a, 114b. Therefore, the sidewalls defining the trenches 114, 114a, 114b would be straight right after all the trenches 114, 114a, 114b are completed if the etching rate of the silicon layer 112 was homogeneous across the substrate 110 in the dry etching.
When the trenches 114, 114a, 114b are formed by the dry etching, however, the etching rate varies depending on, for example, the size of etched features. Specifically, the etching rate decreases as the trench width narrows due to, so-called, micro-loading effect. The micro-loading effect is caused by insufficient supply of etching gasses into a narrow trench. More specifically, the micro-loading effect is caused because the etching gasses can not be sufficiently supplied to the bottom of the narrow trench while a constant amount of the etching gasses are supplied to the entrance of the trench.
For example, in FIGS. 4 and 5, a wide trench 114a that are defined by one of the movable electrode 124 and one of the springs 122 has width W1, which is wide enough not to be susceptible to the micro-loading effect. On the other hand, a narrow trench 114b that are defined by a pair of beams making up the one of the springs 122 has width W2, which is narrow enough to be susceptible to the micro-loading effect. Therefore, during the dry etching, although the etching gasses are sufficiently supplied to the bottom of the wide trench 114a, the etching gasses are not sufficiently supplied to the bottom of the narrow trench 114b. As a result, the etching rate is slower at the bottom of the narrow trench 114b than at the bottom of the wide trench 114a. Thus, when the wide trench 114a is completed, the narrow trench 114b is not completed yet, as shown in FIG. 6A.
To avoid the incomplete etching of the narrow trench 114b caused by the micro-loading effect, the etching is continued until the narrow trench 114b is completed. In that case, however, as shown in FIG. 6B, when the narrow trench 114b is completed, each of the sidewalls defining the wide trench 114a is locally etched by the interface between the semiconductor layer 112 and the insulating intermediate layer 113, so notches are generated on the sidewalls defining the wide trench 114a at the bottom thereof. The phenomenon is called “notching”. Specifically, the “notching” is caused because a charge builds up due to etching ions on the surface of the insulating intermediate layer 113, which is exposed during the dry etching at the bottom of the wide trench 114a. The charged surface repels etching ions to change the ion trajectory angle, so etching ions are deflected towards the sidewall. As a result, the sidewalls are etched by the interface between the semiconductor layer 112 and the insulating intermediate layer 113.
As shown in FIG. 6C, after the trenches 114, 114a, 114b are completed, the movable electrodes 124 and the fixed electrodes 132 are finally formed by dry etching the side walls defining the trenches 114, 114a, 114b at the portions adjacent to the bottoms of the trenches 114, 114a, 114b. However, as shown in FIG. 6B, each of the sidewalls defining the wide trench 114a has a notch, and the shape of the notch significantly deviates with in an etching batch and between etching batches.
As a result, the cross-sectional shape of the springs 122 significantly fluctuates between the sensors and so does the performance of the springs 122. Thus, the acceleration sensors that are manufactured in the conventional process have significant sensor-to-sensor deviation in sensor characteristics.